This paper presents a numerical study of the effect of fine content on the mechanical behavior of gap-graded granular materials using the discrete element method. Triaxial compression tests are performed on different samples with fine contents varied from 0% to 40%. It was found that, starting from 20%, fine content has a visible effect on the shear strength. The optimal fine content is about 30%, at which the shear strength is the best. An investigation into the granular micro-structure showed that the fine particles, on one hand, come into contact with coarse particles, but on the other hand, separate the latter ones as fine content increases beyond 20%. Consequently, the shear stress is transmitted more and more through the coarse-fine contacts but less and less through the coarse-coarse contacts. For fine content ≤ 30%, the coarse-coarse contacts primarily carry the shear stress. Above this optimal fine content, the fine-coarse contacts overtake the coarse-coarse ones. The fine-fine contacts have little contribution to supporting the shear stress. For the studied range of fine content, the coarse particles primarily carry the shear stress, leaving the fine particles under relatively low stresses. Moreover, the fine particles are greatly softened by the shear loading. A classification of binary mixtures depending on their micro-structure was also proposed.

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